I just read this stuff in a very recent thread. Obviously this also depends on your setup and lighting.
Check out Latewoods post. Its the third one down.
http://boards.cannabis.com/showthread.php?t=60300
elcheapo Reviewed by elcheapo on . TDS question When using a TDS meter is there any set ppm that you want your nutrients to be at for certain segments of the grow? Where should the ppm be maintained is basically my question. Rating: 5